1. Field of the Invention
The present invention relates to a method for detecting malfunctioning nozzles of an inkjet printer having a print head, the print head comprising at least one array of at least two nozzles, each nozzle being configured to eject an ink drop which in operation is to result in a dot on the a recording medium. The present invention also pertains to an inkjet printing apparatus having a control unit and a print head, the print head comprising at least one array of at least two nozzles, wherein each nozzle is configured to eject an ink drop which in operation is to result in a dot on a receiving medium, the control unit being configured to control the inkjet printing apparatus.
2. Description of Background Art
Inkjet printers have at least one inkjet print head from which ink is ejected on a receiving medium. An inkjet print head comprises at least one array of nozzles from which amounts of ink are ejected for forming an image on the receiving medium.
A method is known from U.S. Pat. No. 6,089,693 for printing an image by a print head including one or more non-functioning nozzles and a plurality of functioning nozzles. Non-functioning nozzles are identified by a defective nozzle detector being incorporated as part of the print head. It is disclosed that, in addition, a test pattern could be printed identifying to a user which of the nozzles are non-functioning and therefore non-productive. The test pattern includes nozzle identifiers, such as a number, which is printed by each of the functioning nozzles and which identifies a nozzle. The missing number would indicate to the user which of the nozzles is non-functioning.
Another category of non-productive nozzles are misdirecting nozzles. By misdirection of an ink drop ejected from the nozzle towards the receiving medium, the ink drop is not delivered on a spot on the receiving medium, which was intended by the ink jet control unit. EP 0 988 990 describes a method to detect misdirecting nozzles, wherein a test pattern is printed on a recording medium. The test pattern is scanned by sensor means to capture an image of ink drops on the recording medium. The image is processed to determine an average position of the ink drops. The actual positions of the ink drops are compared with the average position to detect deviations due to misdirecting nozzles.
For detecting deviations on the test image sensor means for scanning the image and processor time for calculating average positions are required, since the diameter of the nozzles, which constitute the printing head become small resulting in small ink drops on the receiving medium. Artifacts due to deviations are hardly visible for the human eye.